1. Field of the Invention
The invention relates generally to knee prostheses and, more specifically, to knee prostheses which more closely emulate the anatomy and function of the knee and thereby feature range of flexion, rotation of the tibia relative to the femur, the screw home mechanism, and other structural and functional characteristics of the actual knee joint.
2. General Background of the Invention
Disease and trauma affecting the articular surfaces of the knee joint are commonly treated by surgically replacing the ends of the femur and tibia with prosthetic femoral and tibial implants, and, in some cases, replacing the patella with a patella component. Such surgeries are sometimes referred to as total knee replacement (TKR). In TKR surgery, a surgeon typically affixes two prosthetic components to the patient's bone structure; a first to the patient's femur and a second to the patient's tibia. These components are typically known as the femoral component and the tibial component respectively.
The femoral component is placed on a patient's distal femur after appropriate resection of the femur. The femoral component is usually metallic, having a highly polished outer condylar articulating surface, which is commonly J-shaped.
A common type of tibial component uses a tray or plateau that generally conforms to the patient's resected proximal tibia. The tibial component also usually includes a stem that extends at an angle to the plateau in order to extend into a surgically formed opening in the patient's intramedullary canal. The tibial component and tibial stem are both usually metallic.
A plastic or polymeric (often ultra high molecular weight polyethylene) insert or bearing fits between the tray of the tibial component and the femoral component. This insert provides a surface against which the femoral component condylar portion articulates, i.e., moves in gross motion corresponding generally to the motion of the femur relative to the tibia.
Modern TKR's are tricompartmental designs; they replace three separate articulating surfaces within the knee joint: the patello-femoral compartment and the lateral and medial inferior tibio-femoral compartments. Most currently available TKR's are designed to articulate from a position of slight hyperextension to approximately 115 to 130° flexion. A tricompartmental design can meet the needs of most TKR patients even though the healthy human knee is capable of a range of motion (ROM) approaching 170°. However, there are some TKR patients who have a particular need to obtain high flexion in the knee joint. For many, a TKR that permits patients to achieve a ROM in excess of 130° is desirable to allow deep kneeling, squatting and sitting on the floor with the legs tucked underneath.
Additionally, a common complaint of TKR patients is that the replaced knee does not does function like a normal knee or “feel normal”. The replaced knee does not achieve normal knee kinematics or motion and generally has a more limited ROM than a normal knee. Currently available designs produce kinematics different than the normal knee during gait, due to the complex nature of the knee joint and the motion of the femur and tibia relative to one another during flexion and extension. For example, it is known that, in addition to rotating about a generally horizontal axis during flexion and extension, the tibia also rotates about its longitudinal axis. Such longitudinal rotation is typically referred to as either external or internal rotation, depending on whether reference is being made to the femur or tibia respectively.
Very few currently available designs allow this longitudinal rotation. One known method to allow rotation is a mobile-bearing knee prosthesis. In mobile-bearing knee prostheses, the insert has increased contact with the condyles of the femoral component and rotates on top of the tibial component. However, mobile-bearing knee prostheses are less forgiving of soft tissue imbalance, increasing the incidence of bearing spin-out and dislocation. Another concern is that the mobile-bearing prostheses create an additional interface and underside wear may occur.
Constructing a total knee prosthesis which replicates the kinematics of a natural knee has been an on-going challenge in the orthopaedic field. Several attempts have been made and are well known in the prior art, including those shown in U.S. Pat. Nos. 6,246,697 and 6,325,828.Conventional designs such as these, however, leave room for improvement in simulating the structure and operation of actual knee joints, in at least the aspects of range of motion, internal rotation of the tibia relative to the femur as the knee flexes, and rotation of the tibia relative to the femur in overextension in order to allow the knee to be stabilized more efficiently.